C-3

[Lyndon S.]

And early post on big block exhaust manifolds that the factory plugged, got me wondering what the plugs looked like? Anyone have a picture that they could post are a good descriptions of them?

Kelly

[Joe L.]

Re: C-3

Kelly-----

They look like the attached drawing. The thread is NOT NPT (i.e. standard pipe thread), it's NOT NC (i.e standard coarse thread), and it's not NF (i.e. standard fine thread). It is a special thread called "national pipe straight thread".

[John H.]

Re: C-3

Corvette didn't have the option, but on the '69 Camaro, RPO KD1 (Less Air Injection - for overseas export only) spec'd P/N 3889330 Plug for small-blocks and P/N 444618 for big-blocks.

[Lyndon S.]

Re: C-3

The part now is 14084945 as it has changed 3-4 times.

[Joe L.]

Re: C-3

John-----

I learned something interesting here. As you know, the AIR fittings in the manifolds are a special thread----national pipe straight thread. If one needs to chase or re-thread the manifold fittings, one needs to obtain a special tap for the purpose. The thread size is 1/4" NPSF. However, the thread pitch is 1/4-18, which is the same as NPT or NPTF.

As far as the plugs go, even though the fittings are NPSF, standard 1/4-18 NPT pipe plugs will work just fine. I always thought that the plugs had to be the special straight pipe thread, but they don't. So, I learned something here that I never knew before.

By the way, the GM #3889330 plugs are an external hex head 1/4-18 pipe plug. I don't have a spec on the GM #444618 plugs. However, plugs of this series are usually internal square head pipe plugs and I believe that is the style usually seen on those rather rare cases in which the factory used AIR-style manifolds on 70-71 Corvette big block applications not requiring AIR. The thread size must be 1/4-18 as that is the same for small blocks or big blocks.

I wonder why they specified the external hex head plugs for small blocks, but the internal square head plugs for big blocks?

[Terry M.]

Re: C-3

Those 1970 Mark IV applications with factory installed AIR fittings plugged do use internal square head plugs. I am confident they are national pipe straight thread because the head of the plug sets well into the tapped hole. I have never had occasion to see a factory plugged small block application -- not to say there was not one, just that has not been my experience on early C3s.

I have seen numerous attempts at plugging these holes with a pipe plug. Only a couple of threads catch, unless one deforms the hole to accommodate the taper of the pipe threads. Usually one can tell the use of pipe plugs because there are some threads proud of the tapped hole, again, unless one re-taps the hole to accommodate the taper. If the hole is deformed, I would question the fit of the straight threads if one should ever decide to reinstall the manifold trees.